The present study will evaluate the effects of a nonselective beta- blocking agent, timolol, versus a beta-blocker with intrinsic sympathomimetic activity (ISA), carteolol, on the variation of nocturnal blood pressure, nocturnal cardiac output, nocturnal intraocular pressure (IOP) and visual function. Recent evidence has shown that a high percentage of primary open angle glaucoma (POAG) patients have unusual variations in both blood pressure and IOP. Topical beta antagonist agents are the drugs of first choice for lowering IOP in POAG patients. Although the systemic side effects of these agents are well established, ie reduction in heart rate, blood pressure, etc, they have been generally ignored because systemic cardiovascular factors have not been considered important for the pathogenesis of the disease. However, if various beta- blocker drugs have substantially different effects on cardiovascular factors which in turn influence the progression of the disease, selection of a treatment regiment must include a more critical review of each drug's systemic side effects.